Increased oil consumption has lead to exploration and drilling in difficult geographic locations that were previously considered economically unfeasible. As is to be expected, drilling under these difficult conditions leads to problems that are not present under more ideal conditions. For example, an increasing number of exploratory wells are being drilled in deep water, offshore locations in an attempt to locate more oil and gas reservoirs. These exploratory wells are generally drilled from floating platforms, leading to a set of problems peculiar to that environment.
As in any drilling operation, offshore drilling requires that drilling fluid must be circulated through the drill bit to cool the bit and to carry away the cuttings. This drilling fluid is normally delivered to the drill bit through the drill string and returned to the floating vessel through an annulus formed between the drill string and a large diameter pipe, commonly known as a riser. The riser typically stands between a subsea wellhead assembly and the floating vessel and is sealed against water intrusion.
The lower end of this riser is connected to the wellhead assembly adjacent the ocean floor, and the upper end usually extends through a central located opening in the hull of the floating vessel. The drill string extends longitudinally through the riser and into earth formations defined below the water, and drilling fluids circulates downwardly through the drill string, out through the drill bit, and then upwardly through the annular space between the drill string and the riser, thereby returning to the vessel.
As these drilling operations progress into deep waters, the length of the riser and, consequently, its unsupported weight also increases. Riser structural failure may result if compressive stresses is in the elements of the riser exceed the metallurgical limitation of the riser material. Riser tensioner systems are typically used to avoid this type of riser failure.
Riser tensioner systems are installed onboard the platform, and apply an upward force to the upper end of the riser, usually by means of cable, sheaves, and pneumatic cylinder mechanisms connected between the vessel and the upper end of the riser.
FIG. 1 illustrates one such type of a riser tensioner system as applied to a drilling vessel or drilling platform 1. The drilling vessel or drilling platform 1 comprises a mast 2, to which a drilling string 3 is fastened, which drilling string extends in the direction of the borehole (not shown). The drilling string 3 is virtually completely enclosed by a riser 4. A riser ring 5 is fastened at the top end of the riser 4. Cables 6, by means of which a tensile force can be exerted upon the riser 4, are fastened to the riser ring 5. Two cables 6 are shown in FIG. 1. In the prior art, it is customary to connect four, six, eight or twelve cables to the riser ring. The cable 6 extends from the riser ring 5 by way of sheaves 10, 11 and 12 in the direction of the cable anchor 13. When the drilling vessel or drilling platform 1 moves relative to the earth's surface, for example, as a result of wave action, the drilling vessel or drilling platform 1 will also move upwards relative to the riser 4. Since the sheaves 11 and 12 are situated on either side of a cylinder 14, these movements of the drilling vessel or drilling platform relative to the riser 4 can be absorbed. When the drilling vessel or drilling platform 1 moves relative to the riser 4, the cylinder 14 will be depressed, with the result that the distance between the sheaves 11 and 12 is reduced in the free end of the cable 6 between the sheave 10 and the riser-tensioner 5 will increase. When the drilling vessel or drilling platform 1 moves in the direction of the riser 4, the opposite will occur.
It is important in the use of such riser tensioners that the riser tensioner does not bottom cut during normal operation. In order to avoid this “bottoming out,” the pressure of the pneumatic or hydraulic fluid within the riser tensioner can be increased or decreased, depending upon need. In order to determine the amount of pressure that must be applied to avoid this “bottoming out,” it is important to continually monitor the relative movement between the sheaves 11 and 12 so that movement beyond a desired limit is avoided. For example, in rough sea conditions, it is important to increase the amount of pressure within the cylinder 14 so as to prevent the “bottoming out.”
Throughout the motion of the riser tensioner, the cable is subjected to a great deal of wear and tear. Wear and tear is typically measured in terms of “ton-miles.” To avoid cable failure after excessive usage, it is important to have a determination of the amount of wear applied to the cable over time. This measure will typically be based upon the amount of fluid pressure within the cylinder 14, and the total amount of movement of the cable over the sheaves 11 and 12 over a period of time.
It is an object of the present invention to provide a riser tensioner sensor system which can effectively provide an indication of cable wear.
It is another object of the present invention to provide a riser tensioner sensor which can continuously monitor relative movement of the sheave assemblies with respect to each other.
It is a further object of the present invention to provide a riser tensioner system which provides immediate and reliable feedback to the operator of the offshore platform.
It is still a further object of the present invention to provide a riser tensioner sensor system which is very durable, even in the extreme environmental conditions of the offshore oil platform.
It is a further object of the present invention to provide a riser tensioner sensor assembly which is easy to use, relatively inexpensive and easy to install.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.